In recent years, the digitalization of information managed and stored in business organizations has been increasingly promoted, and the practice of storing information in the paper media is expected to further decline in the future. However, this tendency does not mean the extinction of paper documents, and accordingly the current status is that the amount of paper documents is inevitably increasing, and digital documents and paper documents are managed together. Also, stricter information management systems need to be introduced in order to prevent the leakage of personal information such as customer information managed by business organizations or public institutions, etc.
As a security technique to cope with this situation, digital watermarking is proposed. Digital watermarking is a technique of embedding information such as the names of copyright holders, histories of copying, etc., in image data, document data, audio data, or the like. Embedding information in important data managed in offices or in products supplied from and used out of business organizations makes it possible to prevent the leakage and illegal copying of information and to secure the original data. For these reasons, research on digital watermarking for digital data has been widely conducted, and various products based on the digital watermarking techniques are produced.
However, it is difficult to extract embedded information from binary document data, particularly from the printed form of paper media. The reason for this is that embedded information is prone to be lost due to position deviations that can be caused during printing or scanning, the addition of noise, the loss of high-frequency components as a result of the resolution conversion, etc.
The technique of Patent Document 1 realizes both an improvement in image quality and the embedment of information by utilizing the characteristics of human eyes. However, this technique involves a high probability of losing embedded information in the case when outline portions of figures or characters fade or blur due to copying or when the resolution changes due to facsimile transmissions. The reason for this is that information is embedded mainly in the outlines of figures and characters or in the intervals between them. Additionally, the amount of information that can be embedded depends upon the number of characters.
Patent Documents 2 through 6 disclose techniques of using background texture patterns for digital watermarking. In the methods of Patent Documents 2 and 3, dots constituting background texture patterns are aligned in a particular direction in order to prevent the degradation of detection accuracy that would be caused by the addition of noise, the blurring of outline portions or the like due to printing, copying, or scanning of the paper media. However, these relatively simple background texture patterns can be detected easily by human eyes so that embedded information can be read by third persons and be misused.
In the method of Patent Document 4, the directions of background texture patterns are detected by using the technique of pattern matching, resulting in lower detection accuracy because of geometric deformation such as deformations caused in the process of being copied, etc. Also, the background texture patterns themselves are large, allowing third persons to visually understand the arrangement of the base patterns.
In the method of Patent Document 5, background texture patterns are embedded in document images in portions other than those containing characters. The technique of matching based on a plurality of types of filters is employed for the identification of the patterns. In the method of Patent Document 6, background texture patterns are generated by combining dots with different resolutions. When a paper document is copied, dots with high resolutions are not copied, whereas dots with low resolutions are copied, making it possible to produce documents that allow both the confirmation of the effect of the original document and the identification of the distribution routes.
In the methods of Patent Documents 5 and 6, directions of background texture patterns are detected by using filters. However, these methods involve the same defect as in the Patent Document 4, namely that the background texture patterns themselves are large, allowing third persons to visually understand the arrangement of the base patterns.    Patent Document 1:    Japanese Patent Application Publication No. 2004-289783    Patent Document 2:    Japanese Patent Application Publication No. 2001-346032    Patent Document 3:    Japanese Patent Application Publication No. 2003-101762    Patent Document 4:    Japanese Patent Application Publication No. 2002-305646    Patent Document 5:    Japanese Patent Application Publication No. 2003-209676    Patent Document 6:    Japanese Patent Application Publication No. 2006-005399